In a scanning electron microscope (“SEM”), a primary beam of electrons is scanned upon a region of a sample that is to be investigated. The energy released in the impact of the electrons with the sample causes the liberation of other charged particles in the sample. The quantity and energy of these secondary particles provide information on the nature, structure and composition of the sample. The term “sample” is traditionally used to indicate any work piece being processed or observed in a charged particle beam system and the term as used herein includes any work piece and is not limited to a sample that is being used as a representative of a larger population. The term “secondary electrons” as used herein includes backscattered primary electrons, as well as electrons originating from the sample. To detect secondary electrons, a SEM is often provided with one or more secondary electron detectors.
In a conventional SEM, the sample is maintained in a high vacuum to prevent scattering of the primary electron beam by gas molecules and to permit collection of the secondary electrons. However, when the beam impinges on a non-conducting region of the sample, the sample tends to accumulate an electric charge, which can deflect the primary beam and affect the number of secondary electrons reaching a detector. The polarity of the accumulated charge depends on the type and energy of particles in the primary beam and on the work piece material. Although the electrons in the primary beam are negatively charged, each impinging primary electron may eject more than one secondary electron, leaving the sample positively charged. Several techniques have been proposed to reduce sample charging, including depositing a conductive layer onto the sample, directing low energy electrons toward the sample, as described in U.S. Pat. No. 6,683,320 to Gerlach et al. for “Through-the-lens neutralization for charged particle beam system,” and directing light toward a semiconductor sample to induce photoconductivity to drain the charge.
Another solution is maintaining the sample under a relatively high pressure and is described, for example, in U.S. Pat. No. 4,785,182, to Mancuso et al., entitled “Secondary Electron Detector for Use in a Gaseous Atmosphere.” Such devices are better known as High Pressure Scanning Electron Microscopes (HPSEM) or Environmental Scanning Electron Microscopes. An example is the Quanta 600 ESEM® high pressure SEM from FEI Company. Secondary electrons are accelerated toward an anode and ionize gas particles along the way, with the ionized gas particles being attracted back to the charged sample, away from the anode, and neutralizing the charge.
In an HPSEM, the sample that is to be investigated is placed in an atmosphere of a gas having a pressure typically between 0.1 Torr (0.13 mbar) and 50 Torr (65 mbar), and more typically between 1 Torr (1.3 mbar) and 30 Torr (40 mbar), whereas in a conventional SEM the sample is located in vacuum of substantially lower pressure, typically less than 10−5 Torr (1.3×10−5 mbar). Besides charge neutralization, an HPSEM offers the possibility of forming electron-optical images of moist samples, such as biological samples, and other samples that, under the high vacuum conditions in a conventional SEM, would be difficult to image. An HPSEM provides the possibility of maintaining the sample in its natural state; the sample is not subjected to the disadvantageous requirements of drying, freezing or vacuum coating, which are normally necessary in studies using conventional SEMs and which can alter the sample.
In an HPSEM, secondary electrons are typically detected using a process known as “gas ionization cascade amplification” or “gas cascade amplification,” in which the secondary charged particles are accelerated by an electric field and collide with gas molecules in an imaging gas to create additional charged particles, which in turn collide with other gas molecules to produce still additional charged particles. This cascade continues until a greatly increased number of charged particles are detected as an electrical current at a detector anode. In some embodiments, each secondary electron from the sample surface generates, for example, more than 20, more than 100, or more than 1,000 additional electrons, depending upon the gas pressure and the electrode configuration. Gas cascade amplification detectors typically do not provide as high resolution or as great amplification as conventional, high vacuum detectors, such as an Everhart-Thornley detector, which uses a combination of a scintillator and a photomultiplier.
An HPSEM limits the region of high gas pressure to a sample chamber by using a pressure-limiting aperture (PLA) to maintain a high vacuum in the focusing column. Gas molecules scatter the primary electron beam, and so the pressure limiting aperture is positioned to minimize the distance that the primary electron beam travels in the high pressure region in order to reduce scattering of the primary beam, while providing a sufficient travel distance between the sample and the detector for adequate gas cascade amplification of the secondary electron signal.
An HPSEM as described in U.S. Pat. No. 4,785,182, comprises a vacuum envelope having a pressure limiting aperture, an electron beam source located within the vacuum envelope and capable of emitting electrons, one or more focusing lenses located within the vacuum envelope and capable of directing an electron beam emitted by the electron source through the pressure limiting aperture, beam deflectors located within the vacuum envelope and capable of scanning the electron beam, and a sample chamber including a sample platform disposed outside the high vacuum envelope and capable of maintaining a sample enveloped in a gas at a desired pressure.
Charged particle beams, such as electron beams or ion beams, can also be used to induce a chemical reaction to etch a sample or to deposit material onto a sample. Such processes are described, for example, in U.S. Pat. No. 6,753,538, to Musil et al. for “Electron Beam Processing.” The process of a charged particle beam interacting with a process gas in the presence of a substrate to produce a chemical reaction is referred to as “beam chemistry.”
US Pub. No. 2011/0031394, which is assigned to the assignees of the present application describes several configurations of environmental cells that allow HPSEM operation. The term “environmental cell” is used to mean an enclosure that contains the sample and provides an environment around the sample, typically a different environment than that present in a vacuum chamber in which the environmental cell is located. An environmental cell can solve some of the above problems by enhancing control of the sample environment, reducing the concentration of gaseous impurities present during HPSEM processing, and reducing the volume and inner surface area of the HPSEM process chamber. However, the environmental cell solutions are not fully compatible with a high vacuum SEM. Thus, there is a need for a method and system that provides an ESEM environment within a high vacuum SEM.
Other environmental cells are described in US Pub. No. 2012/0112062, and U.S. Pat. No. 8,093,558. Gas injection systems for use in a high vacuum SEM that include a shroud-type concentrator are described, for example, in U.S. Pat. No. 5,851,413.